True DC current source

ABSTRACT

According to one embodiment, a power converter circuit includes a resonant circuit coupled to an alternating current (AC) voltage source to convert a first AC voltage to a first AC current and an AC to direct current (AC/DC) converter coupled to the resonant circuit, where the AC/DC converter is to convert the AC current to a DC current. The power converter circuit further includes an inverter coupled to the AC/DC converter to convert the DC current to a second AC current, an AC filtering circuit coupled to an output of the inverter, and a load coupled to the output of the inverter to convert the second AC current to a second AC voltage.

RELATED APPLICATIONS

This application is a continuation application of U.S. patentapplication Ser. No. 15/595,896, filed, May 15, 2017, now U.S. Pat. No.10,110,140, which claims the priority of U.S. Provisional PatentApplication No. 62/337,298, filed May 16, 2016. The disclosure of theseapplications is incorporated by reference herein in its entirety.

FIELD OF THE INVENTION

The present invention relates generally to power electronic circuits.More particularly, this invention relates to the generation of a truedirect current (DC) source for power electronic circuits. A true currentsource is characterized by an applying infinite voltage in the case ofan open circuit, just as in the same way a true voltage source ischaracterized by supplying an infinite current during a short circuit.

BACKGROUND

Power conversion, from alternating current (AC) to direct current (DC)and/or from DC to AC is achieved today predominantly by voltage sourceconverters (inverters and rectifiers) having a switching circuit. Forexample, for a rectifier, either a three phase or single phase ACvoltage source supply is rectified to a controlled or uncontrolled DCbus voltage. Inverters in either three phase or single phase powercircuits use a fixed DC bus voltage that may be center-tapped to createa modulated stepped waveform on the AC side using a switching algorithm.The stepped waveform is filtered via an AC filter to create an ACvoltage. The AC voltage is usually regulated by altering the switchingpattern, or by regulating the DC bus voltage, or a combination thereof.The resulting waveform can be connected to a dead load (resistor,inductor, and capacitor) or paralleled with another voltage source,depending on the application.

Typical applications include rectifiers and high current rectifiers (forDC loads), frequency converters, UPSs, solar inverters, battery energystorage systems, etc. The power conversion described above is achievedusing voltage source technology. The same power conversion objectivescan be achieved using current source technology. This paper seeks toexplain a method for the generation of a true current source and how itcan be applied in the domain of AC/DC and DC/AC power conversion.

FIG. 1A is a schematic diagram illustrating a series connected inductorand capacitor, or an LC circuit to transform an AC voltage source to anAC current source at the load terminals, load 207. Referring to FIG. 1A,circuit 200 transforms an AC voltage source into an AC current source atload 207. AC voltage source 201, such as an AC 60 Hz 120V, power sourceprovides a constant voltage to circuit 200. Tuning inductor 203 andcapacitor 205, having a resonant frequency of 1/(2*pi*SQRT(LC)), to thevoltage source frequency, e.g., 60 Hz, as the load resistance tendstoward infinity the load voltage also tends toward infinity. An infiniteelectro motive force (EMF) or voltage and infinite resistance defines acurrent source. Here, L is inductance of inductor 203 and C iscapacitance of capacitor 205.

FIG. 1B is a schematic diagram illustrating a series connected inductorwith a parallel connected capacitor and inductor, or a resonant LCLcircuit to transform an AC voltage source to an AC current source at theload terminals. Referring to FIG. 1B, circuit 250 transforms an ACvoltage source into an AC current source at load 259. AC voltage source251, such as an AC 60 Hz 120V, power source provides a constant voltageto circuit 250. Tuning inductor 253 and capacitor 255, having a resonantfrequency of inverse of a multiplied by the square root of (LC), to thevoltage source frequency, e.g., 60 Hz, drives the circuit to behave witha constant current characteristic. Here, L is inductance of inductor 253and C is capacitance of capacitor 255. Inductor 257, also tuned to thesupply frequency, connected in series with load 259 ensures no currentis drawn from the supply at no load due to parallel resonance. Parallelresonance occurs when a circuit current is in phase with the appliedvoltage of an AC circuit containing an inductor and a capacitorconnected in parallel. The circuit creates a true AC current source atload 259, e.g., a current in resistance 259 that is independent of themagnitude of the resistance of load 259.

Appendix A shows calculations for a general case of a 1200 W AC currentsource of FIG. 1B. The equations prove that if the reactors andcapacitors are designed for equal power and tuned to the supply resonantfrequency the output current does not vary with a change of load, fromzero to nominal ohms for component 259.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention are illustrated by way of example and notlimitation in the figures of the accompanying drawings in which likereferences indicate similar elements.

FIGS. 1A and 1B are schematic diagrams illustrating an AC voltage sourceto AC current source converter.

FIGS. 2A and 2B are schematic diagrams illustrating a single-phase trueDC current source circuit according to certain embodiments of theinvention.

FIG. 2C shows waveforms of circuits as shown in FIG. 2B.

FIG. 3 is a schematic diagram illustrating a true DC current sourcecircuit according to one embodiment of the invention.

FIG. 4 is a schematic diagram illustrating a true DC current sourcecircuit according to another embodiment of the invention.

FIGS. 5A and 5B are schematic diagrams illustrating a DC-AC powerconverter according to certain embodiments of the invention.

FIGS. 5C and 5D show waveforms of circuits as shown in FIGS. 5A and 5B.

FIG. 6 is a schematic diagram illustrating a three-phase DC-AC convertercircuit according to one embodiment of the invention.

FIGS. 7A-7C are schematic diagrams illustrating a three-phase DC-ACconverter circuit according to some embodiments of the invention.

FIG. 8 is a schematic diagram illustrating a three-phase DC-AC convertercircuit having one or more voltage regulators according to oneembodiment of the invention.

FIG. 9 is a schematic diagram illustrating a three-phase DC-AC convertercircuit according to another embodiment of the invention.

FIG. 10 is a schematic diagram illustrating a three-phase DC-ACconverter circuit according to another embodiment of the invention.

FIG. 11 is a schematic diagram illustrating a three-phase DC-ACconverter circuit according to another embodiment of the invention.

FIG. 12 shows examples of regulators that can be used with someembodiments of the invention.

DETAILED DESCRIPTION

Various embodiments and aspects of the inventions will be described withreference to details discussed below, and the accompanying drawings willillustrate the various embodiments. The following description anddrawings are illustrative of the invention and are not to be construedas limiting the invention. Numerous specific details are described toprovide a thorough understanding of various embodiments of the presentinvention. However, in certain instances, well-known or conventionaldetails are not described in order to provide a concise discussion ofembodiments of the present inventions.

Reference in the specification to “one embodiment” or “an embodiment”means that a particular feature, structure, or characteristic describedin conjunction with the embodiment can be included in at least oneembodiment of the invention. The appearances of the phrase “in oneembodiment” in various places in the specification do not necessarilyall refer to the same embodiment.

The present disclosure describes methods to achieve power conversion,which is able to be applied in the same industries and application asdescribed above, using true current source technology. The primary aimis to create a DC current source from an AC voltage network, from whichall the above types of power conversion can be realized.

Voltage source inverter is a DC-AC converter circuit that connects aconstant DC voltage source to an inverter circuit to generate an ACvoltage with adjustable magnitude and/or frequency. Current sourceinverter is a DC-AC converter circuit that connects a constant DCcurrent source to an inverter circuit to generate an AC current withadjustable magnitude and/or frequency. A true voltage source is typifiedsupplying an infinite current during a short circuit. A true currentsource is typified by supplying an infinite voltage during an opencircuit.

According to one aspect of the invention, a DC current source circuitincludes a resonant circuit coupled to an AC voltage source, aninverter, and an optional load. The AC voltage source may be a singlephase or a multi-phase (e.g., three-phase) AC voltage source. Theresonant circuit includes an LC circuit with LC parameters configured toresonate based on an operating frequency of the AC voltage source toconvert an AC voltage into an AC current, effectively converting the ACvoltage source into an AC current source. The LC circuit may be aninductor-capacitor-inductor (LCL) or capacitor-inductor-capacitor (CLC)circuit, connected either in a wye configuration or a deltaconfiguration. The resonant circuit may be a single-phase or multi-phase(e.g., three-phase) resonant circuit. The inverter is configured toconvert the AC current into a DC current, effectively converting an ACcurrent source into a DC current source. The DC current with the loadconverts the DC current into a DC voltage at the load terminals. Theinverter may include a rectifier having a diode bridge. The inverter maybe a single-phase or multi-phase (e.g., three-phase) inverter.

According to one embodiment, a power converter circuit includes aresonant circuit coupled to an AC voltage source to convert a first ACvoltage to a first AC current and an AC to direct current (AC/DC)converter coupled to the resonant circuit, where the AC/DC converter isto convert the AC current to a DC current. The power converter circuitfurther includes an inverter coupled to the AC/DC converter to convertthe DC current to a second AC current, an AC filtering circuit coupledto an output of the inverter, and a load coupled to the output of theinverter to convert the second AC current to a second AC voltage.

According to another aspect of the invention, a power converter circuitincludes a DC current source (e.g., true DC current source) and a DC/ACconverter or inverter. The DC/AC converter or inverter is utilized asthe load that is coupled to the DC current source (e.g., a true DCcurrent source). The DC/AC converter includes a thyristor bridge orinverter and a bank of capacitors coupled to the thyristor bridge. Thebank of capacitors serve to provide reactive power to the AC network(e.g., load plus inverter) and to assist in commutation of thethyristors. The thyristor bridge is to convert a DC current generatedfrom the DC current source to an AC current. The thyristor bridge may bea three-phase bridge to convert the DC current into a three-phase ACcurrent.

According to one embodiment, a DC current source circuit includes aresonant circuit coupled to a three-phase AC voltage source to convertan AC voltage to an AC current. The three-phase resonant circuitincludes a first inductor-capacitor-inductor (LCL) circuit coupled to afirst terminal of the AC voltage source corresponding to a first phase,a second LCL circuit coupled to a second terminal of the AC voltagesource corresponding to a second phase, and a third LCL circuit coupledto a third terminal of the AC voltage source corresponding to a thirdphase. The DC current source circuit further includes a rectifiercoupled to the resonant circuit to convert the AC current to a DCcurrent. The rectifier includes a diode bridge having six diodes and anoutput of the rectifier is coupled to a load.

FIG. 2A is a schematic diagram illustrating a single-phase true DCcurrent source circuit according to one embodiment of the invention. Inone embodiment, circuit 300 includes AC voltage source 301, a resonantcircuit 302, and an AC/DC converter. AC voltage source 301 provides anAC voltage source to circuit 300. Resonant circuit 302 may be an LCresonant circuit (e.g., LC resonant circuit such as LCL or CLC resonantcircuit) where the LC parameters are configured to cause the circuit toresonate at a frequency equal to the frequency of the AC voltage source301. Resonant circuit 302 includes a first reactance 303, a secondreactance 307, and a third reactance 305. The first reactance (inductor303) is coupled to a first terminal of AC voltage source 301. The secondreactance (inductor 307) is coupled to the first reactance (inductor303) by a wye connection (e.g., four-wire circuit) or delta connection(e.g., three-wire circuit). The third reactance (capacitor 305) iscoupled between the wye connection and a second terminal of AC voltagesource 301. The rectifier or AC/DC converter 309 has a first inputcoupled to the second reactance (inductor 305) and a second inputcoupled to a second terminal of AC voltage source 301. Load 311 iscoupled between a first and a second output of rectifier 309 to receivea constant DC current.

In electrical engineering, three-phase electric power systems have atleast three conductors carrying AC voltages that are offset in time byone-third of the period. A three-phase system may be arranged in delta(A) or star (Y) (also denoted as wye). A wye system allows the use oftwo different voltages from all three phases, such as a 230/400 V systemwhich provides 230 V between the neutral (center hub) and any one of thephases, and 400 V across any two phases. A delta system arrangement onlyprovides one voltage magnitude, however it has a greater redundancy asit may continue to operate normally with one of the three supplywindings offline, albeit at 57.7% of total capacity.

In one embodiment, referring back FIG. 2A, rectifier 309 may be a diodebridge, a rectifier bridge, or any switching circuits. In someembodiments, rectifier 309 may be a half wave or a full wave rectifier.In another embodiment, the first reactance and the second reactance arecapacitors and the third reactance is an inductor. In anotherembodiment, load 311 may be a capacitive, reactive, resistive load, or aparallel/series combination thereof, or may be a subsequent stage of anelectronic circuit, or a battery or DC source. By appropriately sizingthe components to tune the LCL (inductor 303, capacitor 305, andinductor 307) resonant circuit to parallel resonance, the circuitdelivers a true AC current through inductor 307 and therefore a true DCcurrent to load 311. The DC current at load 311 is relatively constantsuch that the real power of load 311 (calculated by DC voltagemultiplied by DC amperes) is a function of DC voltage across load 311.In other words, at the load terminals, in absence of any controlcircuitry, the DC voltage fluctuates as the impedance 311 is varied,while the DC current remains relatively constant.

The output resistor 311 can be increased or decreased from its nominalvalue. Since the circuit is a true current source the output currentwill remain constant and an increase of resistor 311 will result in anincrease in output DC voltage and an increase in input AC current fromthe AC source in the same proportion above nominal value, and anincrease in overall power. The inverse is true for a decrease in nominalresistance 311. An increase (or boost circuit topology) in DC voltagecan be useful in some applications, and this is a method of using acurrent source to set the DC voltage to a desired nominal value ratherthan using more conventional means such as a transformer or DC/DCconverters. A decrease in DC voltage (or buck topology) can also beuseful for some applications such as high current rectifiers. The sameprinciples apply for 3 phase circuits, described below.

In one embodiment, a regulator may be coupled to capacitor 305 (e.g.,across in parallel with capacitor 305) to regulate the DC current, whichin turn regulates the power or DC voltage at load 311. Alternatively,the regulator may be coupled to an input of rectifier 309. The regulatormay be a single phase regulator implemented in a variety of ways asshown in FIG. 12.

FIG. 2B is included to illustrate clearly the response of the circuit toa load variation of load 311. FIG. 2B is identical to FIG. 2A exceptthere is a switch across the DC load 311 through an impedance of equalvalue to 311. Closing the switch will result in a half of the loadresistance, and therefore a decrease in DC voltage for a constant DCcurrent. At the instance switch S is closed the two output resistancescombine to produce a resistance of half the value. Waveform 264 of FIG.2C shows the current in L307. Waveform 265 shows the DC current into thecombined load, waveform 266 and 267 show the current in the diode legs.Waveform 268 shows the DC voltage of the load. Switch S closes at 500 ms(point 263). Note that the essence of this embodiment of the inventionis that it is a DC current source. Regardless of the position of switchS the DC current remains unchanged in steady state (waveform 265). Theoutput DC voltage adjusts to a new steady state value according to ohmslaw (see point 263 of waveform 268).

Similar to FIG. 2A, in one embodiment, a regulator may be coupled tocapacitor C5 (e.g., across in parallel with capacitor C5) to regulatethe DC current, which in turn regulates the power or DC voltage at loadR10. Alternatively, the regulator may be coupled to an input ofrectifier 309. The regulator may be a single phase regulator implementedin a variety of ways as shown in FIG. 12.

FIG. 3 is a schematic diagram illustrating a circuit having threesingle-phase true DC output current sources according to one embodimentof the invention. Referring to FIG. 3, the circuit is useful in 3 phasesource applications where loads 411, 421, 431 are unequal, or individualcontrol of DC current in each load is necessary. In one embodiment,circuit 400 includes three-phase AC voltage source 401, a three-phaseresonant circuit, and an inverter having three single-phase inverters.Resonant circuit includes reactance components 403, 405, 407 for a firstphase, reactance components 413, 415, and 417 for a second phase, andreactance components 423, 425, and 427 for a third phase. The inverterincludes rectifiers 409, 419, and 429, one for each of the three phases.

In one embodiment, a first reactance (inductor 403) is coupled to afirst terminal of three-phase AC voltage source 401. A second reactance(inductor 407) is coupled to the first reactance (inductor 403) by afirst wye connection. Center-tapped wye network 404, or second wyeconnection, having a third (capacitor 405), a fourth (capacitor 415),and a fifth (capacitor 425) reactance has a first terminal coupled tothe first wye connection. Circuit 400 further includes a firstsingle-phase rectifier (rectifier 409) having a first input coupled tothe second reactance (inductor 407) and a second input coupled to acenter tap of network 404. Load 411 is coupled between a first and asecond output of rectifier 409 to receive a constant direct current.

Circuit 400 further includes a sixth reactance (inductor 413) coupled toa second input terminal of three-phase AC voltage source 401, a seventhreactance (inductor 417) coupled to the sixth reactance (inductor 413)by a third wye connection, coupled to a second terminal of network 404,and a second single-phase rectifier (rectifier 419) having a first inputcoupled to the seventh reactance (inductor 417) and a second inputcoupled to a center tap of network 404. Load 421 is coupled between afirst and a second output of rectifier 419 to receive a constant directcurrent.

Circuit 400 further includes an eighth reactance (inductor 423) coupledto a third input terminal of three-phase AC voltage source 401, a ninthreactance (inductor 427) coupled to the eighth reactance (inductor 423)by a fourth wye connection, coupled to a third terminal of network 404,and a third single-phase rectifier (rectifier 429) having a first inputcoupled to the ninth reactance (inductor 427) and a second input coupledto a center tap of network 404. Load 431 is coupled between a first anda second output of rectifier 429 to receive a constant direct current.In another embodiment, loads 411, 421, and 431 may be capacitive,reactive, resistive loads, or a parallel/series combination thereof, ora battery, or may be subsequent stages of an electronic circuit.

In one embodiment, the first, second, and third rectifiers may be diodebridge rectifiers, thyristor bridge rectifiers, pulse-width modulation(PWM) rectifiers or any switching circuits. In another embodiment, thefirst, second, sixth, seventh, eighth, and ninth reactances arecapacitive reactances or capacitors, and the third, fourth, and fifthreactances are inductive reactances or inductors.

In one embodiment, a regulator may be coupled to capacitor network 404to regulate the DC current, which in turn regulates the power or DCvoltage at the loads 411, 421, and 431. Alternatively, the regulator maybe coupled to an input of rectifiers 409, 419, and 429. The regulatormay be a three-phase regulator implemented in a variety of ways as shownin FIG. 12.

FIG. 4 is a schematic diagram illustrating a true DC output currentcircuit having a three-phase input voltage source according to oneembodiment of the invention. Circuit 500 includes three-phase AC voltagesource 501. Circuit 500 includes AC voltage source 501, a three-phaseresonant circuit, and a three-phase inverter 509. The three-phaseresonant circuit includes reactance components 503, 505, and 507 for afirst phase, reactance components 513, 515, and 517 for a second phase,and reactance components 523, 525, and 527 for a third phase.

In one embodiment, a first reactance (inductor 503) is coupled to afirst terminal of three-phase AC voltage source 501. A second reactance(inductor 507) is coupled to the first reactance (inductor 503) by afirst wye connection. Three-terminal network 504 includes a third(capacitor 505), a fourth (capacitor 515), and a fifth (capacitor 525)reactance connected in a delta configuration. Capacitors 505, 515, 525may be connected in wye configuration (not shown). A first terminal ofthe three-terminal network is coupled to the first wye connection ofcircuit 500. Circuit 500 further includes AC/DC three-phase rectifier509 having a first input coupled to the second reactance (inductor 507).

Circuit 500 further includes a sixth reactance (inductor 513) coupled toa second input terminal of the three-phase AC voltage source 501, aseventh reactance (inductor 517) coupled to the sixth reactance(inductor 513) by a second wye connection, coupled to a second terminalof the three-terminal network. Circuit 500 further includes an eighthreactance (inductor 523) coupled to a third input terminal of thethree-phase AC voltage, a ninth reactance (inductor 527) coupled to theeighth reactance (inductor 523) by a third wye connection, coupled to athird terminal of the three-terminal network. Load 511 is coupledbetween a first and a second output of AC/DC three-phase rectifier 509to receive a true constant direct current. In one embodiment, AC/DCthree-phase rectifier 509 may be a diode bridge rectifier, a thyristorbridge rectifier, PWM rectifier or any switching circuits. In anotherembodiment, the first, second, sixth, seventh, eighth, and ninthreactances are capacitive reactances and the third, fourth, and fifthreactances are inductive reactances.

In one embodiment, a regulator may be coupled to capacitor network 504to regulate the DC current, which in turn regulates the power or DCvoltage at the load 511. Alternatively, the regulator may be coupled toan input of rectifier 509. The regulator may be a three-phase regulatorimplemented in a variety of ways as shown in FIG. 12.

Embodiment of Current Source Inverter as a DC Load

This section describes a method for how a true DC current source, asdescribed above, can be inverted to create an AC voltage source. The DCsource is a true current source. The DC current is inverted by athyristor inverter and flows into an AC impedance consisting of acapacitor bank and resistor/inductor network. The AC capacitor bank ispresent to provide reactive power to the AC output network and assist incommutating the thyristors of the inverter. With appropriately sizedelements on the AC side and conventional commutation sequence of thebridge thyristors, the DC current inverts to AC current and creates anAC voltage. The voltage can be three phase or single phase, depending onthe inverter and load connected. The magnitude of the synthesized ACvoltage, for a fixed DC current, is dependent on the sizing of the ACload (Resistor, inductor, capacitor). If the DC current is assumed to beconstant, the DC voltage will depend on the magnitude of the real powerof the connected load. The load reactive power will circulate betweenthe AC capacitor bank and the AC inductance at no load (infinite loadresistance), no real power exists therefore the DC voltage of theinverter is almost zero (supplying only stray losses). As the ACresistive load increases the DC voltage increases such that the power onthe DC side of the inverter equals the AC real power (plus straylosses). At all times the DC current in the inverter remains unchanged,and the AC voltage is product of AC current multiplied by the impedance,Z, of the inverter.

In one embodiment, a second stage of the electronic circuit includes aDC-AC inverter coupled to a three-phase capacitor bank such that theDC-AC inverter converts a DC input signal to a three-phase AC outputsignal. The inverter may be thyristor inverter using force commutationor AC capacitor assistance, or it may be PWM insulated gate bipolartransistor (IGBT) inverter or any other switch plus a filter. The outputof the inverter is connected to a three-phase load, which may be asubsequent stage of the electronic circuit.

FIG. 5A is a schematic diagram illustrating a DC-AC power converteraccording to one embodiment of the invention. The DC-AC power convertermay be a second stage of a two-stage electronic circuit to complete atrue current source inverter electronic circuit, for example, as shownin FIG. 6. For example, DC current source 101 includes a true DC currentsource provided by an AC-DC power converter electronic circuit, such aselectronic circuits of FIGS. 2A-2B and 3-4. Referring to FIG. 5A,electronic circuit 100 includes a true DC current source 101 connectedto the DC poles of three-phase thyristor inverter or thyristor bridge102 operating as an inverter (e.g., a three-phase inverter). DC currentsource 101 may represent any of the true DC current sources as describedabove. AC capacitor bank 103 is connected to AC terminals of thethyristor inverter 102. Three-phase load 104 is connected in parallelwith capacitor bank 103. By commutating, or switching on and off, sixthyristors of thyristor inverter 102 in a conventional progression,i.e., THY1, THY3, THY5, for the top row of devices and THY4, THY6, THY2for the bottom row of devices, a DC current is commutated to create anAC current to form a three-phase AC voltage across AC terminals ofinverter 102, each phase voltage 120 degrees apart. The phase to phasevoltage on the AC side of the inverter bridge provides the means ofcommutation of the thyristors (AC capacitor bank assisted commutation).

For example, gate terminals of thyristors of thyristor inverter 102,such as THY1, may be coupled to a pulsing circuit or gate driver orcontroller (not shown) configured to pulse at predetermined intervals tocontrol the thyristors' switching time to output a pulse width modulated(three-phase AC or sinusoidal signals) at the output terminals. The gatedriver switching interval regulates the output frequency and phaseangles between the three-phase AC signals. Note that no feedback circuitis required to produce the output sinusoidal voltage waveform. The gatedriver/controller may include a machine-readable medium (e.g., memory)storing a switching or firing algorithm to turn on or turn off thethyristors according to a predetermined firing scheme.

The amplitude of AC voltage across load 104 is determined by theimpedance of capacitor bank 103, load 104, and a magnitude of the DCcurrent of source 101. The DC current of DC current source 101 remainsrelatively unchanged, irrespective of load changes in 104. A load changeis characterized by a change in DC voltage. In other embodiments, load104 may be capacitive, resistive, or a subsequent stage of an electroniccircuit. In other embodiments, the DC source current may be variable,providing a method to control the AC voltage. Capacitor bank 103 may beconnected in delta or wye configuration. Capacitor bank 103 may be tunedto provide reactive power compensation, i.e., capacitor bank 103corrects power factor to increase real power delivered to load 104. Inanother embodiment, the power converter of FIG. 5A (102) may be a singlephase thyristor inverter with 180 degrees of conduction angles for eachpair of inverters of the single phase thyristor inverter as shown inFIG. 5B.

In one embodiment, a three-phase transformer may be coupled between ACcapacitor bank 103 and load 104 of FIG. 5A for the purpose of isolatingload 104 from the AC capacitor bank. A single-phase transformer may becoupled between the AC capacitor bank and the load in FIG. 5B to isolatethe AC capacitor bank and the load.

FIGS. 5C and 5D show the waveforms associated with the circuit of FIG.5A. Note that there is no change of DC current in this example. The DCcurrent is constant. For illustration the switch closes at 900 ms (point162), showing the transient of the various measurements of AC and DCvoltage and current when the circuit switches from a no load conditioninto a full load condition. The thyristor switches in this case areswitched in an open circuit control fashion, in a basic conventionalsequence described above, see waveforms 167, 168). Closing of switch Sat 900 ms does not have an effect on any of the current waveforms (166,167, 168, 169, 170, and 171). Since the DC current is a true constantsource, the DC and AC current waveforms remain unaltered (166, 169, 170,and 171). The AC voltage generated is 3 phase, 120 degrees apartsinewave (see 186, 187, 188), synthesized by the current passing throughthe impedance of the AC capacitor bank. The voltage waveform across thecapacitor is an integral of the current waveform from the formulav(t)=(1/C)*integral(i(t)*dt).

The DC voltage, waveform 185, experiences a step at 900 ms (point 181).The DC voltage and current are now both nonzero values and the source isdelivering real power (watts) to the load, 104. The AC voltages,waveforms 186, 187, 188 experiences an alteration of waveform because ofthe parallel connected load impedance which shares the AC current withthe capacitor bank in ratio of their complex ohmic values. Waveform 189shows the anode-cathode voltage of thyristor 1. As can be seen thereverse voltage duration, after device turn off, applied to thethyristor before and after 900 ms are not equal. BetaNL (184) is largerthan BetaFL (183), meaning for the same AC voltage the DC voltage musthave increased. This occurs naturally (no control loop) in order torespect the laws of thyristor rectifier and inverter circuits inrelation to the relationship between their respective AC and DCquantities. (further explanation and definitions of this are beyond thispaper). Maximum and minimum beta quantities must be respected in orderto produce a reliable converter design and would be taken into accountwhen sizing the elements of this inverter.

In another embodiment, referring back to FIG. 4, load 511 may be acapacitive, reactive, resistive load, battery, DC source or acombination thereof. In some embodiments, load 511 may be a second stageof an electronic circuit. For example, load 511 may include a DC/ACconverter. The DC/AC converter may be force commutated thyristor, ACcapacitor bank assisted thyristor commutation, or PWM IGBT, gateturn-off (GTO), or other turn off device switch plus a filter, or anyother switches.

FIG. 6 is a block diagram illustrating an example of an AC/DC plus DC/ACconverter according to one embodiment of the invention. FIG. 1600 may besingle phase or three phase or a mixture of both. First stage 1620 maybe single phase and second stage 1630 may be three phase, or first stage1620 may be three phase and second stage 1630 may be single phase.Referring to FIG. 6, the converter 1600 includes first stage circuit1620 and second stage circuit 1630. First stage circuit 1620 mayrepresent any of the AC voltage source to DC current source circuit asdescribed above, including the circuits as shown in FIGS. 2A-2B, 3-4,and 5A-5B. First stage circuit 1620 includes an AC voltage source 1601,a resonant circuit 1602, an optional regulator 1603, an AC/DC converter1604, and an optional DC filter 1605. Resonant circuit 1602 may be anLCL or CLC resonant circuits as described above.

Regulator 1603 may be positioned after the resonant circuit 1602 (shown)or across the wye point of resonant circuit 1602. That is to say, acrosscapacitor 305 in FIG. 2A or across the network 504 in FIG. 4. Regulator1603 may be series connected or shunt connected. Regulator 1603 may be aback to back SCR connected in series or shunt, with an optional seriesinductance. Regulator 1603 may be a 3 phase thyristor bridge or 3 singlephase thyristor bridges connected in series or in shunt. Regulator 1603may be back to back IGBTs connected in series or shunt with an optionalseries inductance. Regulator 1603 may be PWM back to back IGBTs plus afilter, connected in series or in shunt. Regulator 1603 may be any otherback to back connected device (switch), connected in series or shunt.Examples of these regulators are shown in FIG. 12. AC/DC converter 1604may include a rectifier such as diode bridge rectifier(s). 1604 may be aPWM rectifier plus AC filter. DC filter 1605 may include one or moreinductors and DC capacitors connected in series and shunt. In oneembodiment, second stage circuit 1630 includes a DC/AC converter orinverter 1606, an optional AC filter 1607, and load 1608. DC/ACconverter 1606 may include a thyristor bridge that is force commutatedor AC capacitor bank assisted (line commutated). 1606 may be a PWM IGBTinverter, or any other turn off device such as GTO. 1607 may be acapacitor bank or filter. The load may or may not include a seriestransformer. In one embodiment, a transformer may be positioned betweenAC filter 1607 and load 1608 for isolation purpose.

Referring now to FIG. 7A, circuit 600 includes two stages of anelectronic circuit. The first stage may represent first stage 1620 andthe second stage may represent second stage 1630 of FIG. 6. The firststage of circuit 600 may be an AC/DC converter circuit 500 of FIG. 4.The second stage of circuit 600 may include DC filter 601 coupled toDC/AC converter or thyristor inverter 603 (such as inverter 102 of FIG.5A) converting DC input signals to AC output signals. Load 607 iscoupled to the output terminals of circuit 600, and three-phasecapacitor bank 605 is coupled between the output terminals of DC/ACconverter 603, and load 607.

In one embodiment, DC/AC converter 603 is a three-phase six pulsethyristor bridge, or an insulated-gate bipolar transistor (IGBT)inverter using PWM, or any switching circuit. In another embodiment,capacitor bank 605 includes three capacitive reactances connected in adelta configuration. In another embodiment, capacitor bank 605 includesthree capacitive reactances connected in a wye configuration. In anotherembodiment, load 607 may be a capacitive, reactive, resistive load, or acombination thereof, or may be a subsequent stage of an electroniccircuit. In some embodiments, inverter 603 may be a diode converter, athyristor converter, or any switching circuits. In some embodiments,inverter 603 may be a full wave or a half wave rectifier. In oneembodiment, DC filter 601 includes an inductive reactance. In anotherembodiment, DC filter 601 is a pass through wire, e.g., no filtering.

With the techniques described throughout this disclosure, some typicalshortcomings of thyristor inverters may be avoided by using a constantDC current rather than constant DC voltage. Some of the advantagesinclude that the output voltage frequency is controlled easily byaltering the commutating period. Adjusting the DC current is a simplemeans of control and regulation of the output voltage. Frequency andphase are determined by the period and pattern of the firing sequence.By appropriately sizing the components (AC and DC) commutation failureshould be avoided. If failure does happen the fault current is limitedby the DC source and therefore can be recovered. Since the inverter isboost topology adjustment of the capacitor bank can be made to selectthe correct AC load voltage for the load for a specific application,possibly eliminating output isolation transformer. Input THID and PF aregood regardless of load due to the tuned resonant circuit. Thetechniques can be utilized in AC voltage source of variable frequency,phase, or magnitude (or all), such as variable speed drive, UPS, andSFC. For UPS the variable DC bus voltage will need a variable DC source.

In one embodiment, a regulator may be coupled to or across capacitornetwork 504 or alternatively, the regulator may be coupled an input ofrectifier 509 to regular the DC current which in turns regulate the DCvoltage supplied to the second stage. In another embodiment, atransformer may be positioned between capacitor bank 605 and load 607for isolation purposes.

DC Current Source Conversion to DC Voltage Source

Referring now to FIG. 7B, circuit 600 includes a first stage circuit anda second stage circuit. The first stage circuit may represent firststage 1620 and the second stage circuit may represent second stage 1630of FIG. 6. Circuit 600 may include DC capacitor bank 701 such that DCcapacitor bank and DC filter 601 are configured in aninductor-capacitor-inductor wye or T network. The introduction of a DCcapacitor bank decouples the AC load voltage to the AC supply voltage,or the first and second stages. This can be advantageous if the inputand output frequencies are not equal. Also, the DC capacitor bank willprovide a DC voltage of lower ripple than a purely inductive DCfiltering. If the DC current source can be regulated, the voltage acrossthe DC capacitor bank can be set to any desired voltage value.

By appropriately designing and sizing the elements of a true currentsource rectifier described previously (such as circuits of FIGS. 3-5, ora first stage circuit of FIGS. 6-7) and according to appendix A toregulate a DC output current to set a desired DC voltage across theoutput, conventional equations governing the ratio between AC and DCvoltage for voltage source diode, thyristor, and/or IGBT rectifiers canbe ignored, allowing more flexibility in power converter circuitdesigns, especially for backup power supplies where a regulated DCenergy source is required and is connected in paralleled with another DCsource of any voltage. Being able to boost or buck the output voltage byregulating the DC true current source provides an alternative means ofvoltage regulation and alleviates the need for more conventionaltechnologies such as isolation transformers, autotransformers or DC/DCconverters. This can be considered a new kind of rectifier whereselection of the components LCL and their ratings can transform a lowvoltage AC network into a high voltage DC network without the use ofconventional means such as isolation transformers, phase controlbridges, DC/DC converters, and autotransformers.

For example, in a typical 400V AC supply system the maximum no load DCvoltage that can be obtained by a conventional three-phase diode bridgewithout the use of a supply transformer is typically around 540 VDC atno load. A thyristor rectifier is limited to the same maximum voltagebut can obtain a large DC voltage range by phase control of theswitching sequence firing the thyristor devices. However this willresult in poor power factors, voltage regulations, and/or harmonics. ADC voltage generated by a true DC current source power converter,without the use of any power transformers, as described above does nothave any limitations in maximum DC voltage. Also, the DC voltage may beregulated and controlled over a large range while at the same time doesnot pose poor the input power factors and harmonics problems ofconventional bridges. Proposed methods of regulation and control aredealt with in the next chapter.

This method uses a true current source passed through a dc load toachieve the desired DC voltage. This means conventional means of usingtransformers and sophisticated power electronics (such as DC-DCconverters) are all avoided. Additionally the typical supply elementsthat are judged by supply authorities, such as THID and THVD and PF arewell within statutory limits. Conventional methods would not achievethis without harmonic filters and power factor correction. Thetechniques can be utilized UPS, SOLAR, or any application that requiresa DC energy source such at battery, ultra capacitors and solar.

In one embodiment, a regulator may be coupled to or across capacitornetwork 504 or alternatively, the regulator may be coupled an input ofrectifier 509 to regular the DC current which in turns regulate the DCvoltage supplied to the second stage. In another embodiment, atransformer may be positioned between capacitor bank 605 and load 607for isolation purposes.

FIG. 7C is a schematic diagram illustrating a three-phase DC-ACconverter circuit according to another embodiment of the invention.Referring to FIG. 7C, the circuit is similar to the ones as shown inFIGS. 7A and 7B, except that the inverter is an IGBT inverter. Inaddition, the DC capacitor bank includes DC capacitor bank 701A and DCcapacitor 701B in series. The middle point of DC capacitor banks701A-701B is connected to a common point of load. A regulator may becoupled to or across capacitor network 504 or alternatively, theregulator may be coupled an input of rectifier 509 to regular the DCcurrent which in turns regulate the DC voltage supplied to the secondstage. In another embodiment, a transformer may be positioned betweencapacitor bank 605 and load 607 for isolation purposes.

Regulation of AC and DC Elements

In some embodiments, one or more regulators may be coupled at variouslocations of the power converter circuits/system to regulate a voltageor a current at the various locations. Referring to FIG. 8, circuit 800may represent circuit 600 of FIG. 7A. Circuit 800 may include any ofregulators 801-807 coupled at various locations of circuit 800 toregulate a current flow by means of series or shunt connection. A shuntconnection prevents current generated by the source reaching the load bydiverting it through a parallel path. In one embodiment, a three-phaseregulator, such as regulator 801, is coupled to the first stage ofcircuit 600, at the three terminals of network 504. In one embodiment,three-phase regulator 803 is coupled to inputs of rectifier 509.

In another embodiment, a single-phase regulator, such as regulator 805,is coupled to outputs of rectifier 509. In one embodiment, a regulator,such as regulator 805, may be coupled to outputs of a DC filter (notshown), such as DC filter 601 of FIG. 7A. In another embodiment, athree-phase regulator, such as regulator 807, is coupled to inputterminals of capacitor bank 605 with the beta of inverter 603 fixed(this is required due to the fixed DC voltage, to produce a regulated ACvoltage). Using regulator 807 as an inductive device diverts currentthat would otherwise pass through to the inverter capacitor bank 605 toregulate the AC voltage at the terminals of inverter 603. An example ofregulator 807 may be a 6 pulse thyristor bridge that operates at a fixedalpha angle of 90 degrees from its supply voltage. Since the inverter isregulating AC voltage, the excess reactive current that is notcirculating in the load or drawn by the inverter will be forced intoregulator 807. In another embodiment, a transformer may be positionedbetween capacitor bank 605 and load 607 for isolation purposes.

FIGS. 9 and 10 show alternative designs of the circuits according tocertain embodiments. Referring to FIG. 9, three sets of thyristorbridges are utilized to form an inverter, where each set of thethyristor bridges may be considered as an inverter for a particularphase. This circuit has the advantage that phase to neutral voltage canbe regulated independently for all three phases. This may be a necessityin the case of unbalanced loads. Referring to FIG. 10, three set ofdiode bridges are utilized to form a rectifier, where each set of thediode bridges may be considered as a rectifier for a particular phase.By appropriately sizing the components to tune the first stage LCLresonant circuits to resonate frequencies, higher frequency harmonicscan be minimized or eliminated.

FIG. 11 is a schematic diagram illustrating a three-phase DC-ACconverter circuit according to another embodiment of the invention. Inthis embodiment, 3 single phase capacitor banks are used and atransformer. The transformer shown is 3 single phase, interconnected,however a single 3 phase transformer may be used in its place. Note thatalthough not shown in FIGS. 9-11, a regulator may be coupled across thecapacitor network of the three-phase resonant circuit or alternatively,the regulator may be coupled to an input of the inverter. In addition, atransformer may be implemented between the AC capacitor bank and theload for isolation purposes.

In the foregoing specification, embodiments of the invention have beendescribed with reference to specific exemplary embodiments thereof. Itwill be evident that various modifications may be made thereto withoutdeparting from the broader spirit and scope of the invention as setforth in the following claims. The specification and drawings are,accordingly, to be regarded in an illustrative sense rather than arestrictive sense.

What is claimed is:
 1. A power converter circuit, comprising: a resonantcircuit coupled to a first alternating current (AC) voltage source toconvert a first AC voltage of the first AC voltage source to a first ACcurrent, wherein the resonant circuit is configured to resonate at anoperating frequency of the first AC voltage source to convert the firstAC voltage source to a first AC current source, wherein parameters ofcomponents of the resonant circuit are specifically selected to have aresonant frequency identical or similar to the operating frequency ofthe first AC voltage source, and wherein the first AC current sourceproduces the first AC current as a constant AC current; a rectifiercoupled to the resonant circuit, wherein the rectifier is to convert thefirst AC current source to a direct current (DC) current source; aninverter coupled to the rectifier to convert the DC current source to asecond AC current source; an AC filtering circuit coupled to an outputof the inverter; and a load coupled to the output of the inverter toconvert the second AC current source to a second AC voltage source. 2.The power converter circuit of claim 1, wherein the second AC current isa constant AC current.
 3. The power converter circuit of claim 1,wherein the resonant circuit comprises an inductor-capacitor-inductor(LCL) circuit or a capacitor-inductor-capacitor (CLC) circuit configuredin a wye configuration or a delta configuration.
 4. The power converterof claim 3, further comprising a first regulator coupled to a capacitorof the LCL circuit or the CLC circuit.
 5. The power converter circuit ofclaim 3, further comprising a second regulator coupled to an inductor ofthe LCL circuit or the CLC circuit.
 6. The power converter circuit ofclaim 1, further comprising a third regulator coupled to an input of therectifier to regulate the first AC current.
 7. The power convertercircuit of claim 1, further comprising a fourth regulator coupled to anoutput of the rectifier.
 8. The power converter circuit of claim 1,wherein the first AC voltage source is a three-phase AC voltage sourcehaving three terminals, each terminal corresponding to one of threephases, and wherein the resonant circuit comprises three LCL circuits orthree CLC circuits.
 9. The power converter of claim 8, wherein each ofthe LCL or CLC circuits is coupled to one of the three terminals of thefirst AC voltage source.
 10. The power converter circuit of claim 8,further comprising three regulators, wherein each of the regulators iscoupled to a capacitor or an inductor of one of the LCL or CLC circuits.11. The power converter circuit of claim 1, wherein the rectifiercomprises a diode bridge, a thyristor bridge, or a pulse-widthmodulation (PWM) circuit.
 12. The power converter circuit of claim 1,wherein the first AC voltage source is a three-phase AC voltage sourcehaving three phases, and wherein the rectifier comprises threerectifiers, each of the rectifiers corresponding to one of the threephases.
 13. The power converter circuit of claim 12, further comprisinga three-phase regulator, wherein the three-phase regulator is coupled toinputs of the rectifiers.
 14. The power converter circuit of claim 13,wherein the thyristors or IGBTs are switched on and off according to apredetermined sequence.
 15. The power converter circuit of claim 1,wherein the inverter comprises a single phase thyristor bridge havingfour thyristors or an insulated gate bipolar transistor (IGBT) bridgehaving four IGBTs.
 16. The power converter circuit of claim 1, whereinthe second AC voltage source is a three-phase AC voltage source, whereinthe inverter comprises a three-phase thyristor bridge having sixthyristors or a three-phase IGBT bridge having six IGBTs.
 17. The powerconverter circuit of claim 1, wherein the second AC voltage source is athree-phase AC voltage source having three phases, wherein the invertercomprises three individual inverters, each individual invertercorresponding to one of the three phases.
 18. The power convertercircuit of claim 17, wherein each of the three individual invertersincludes a thyristor bridge having four thyristors or a three-phase IGBTbridge having four IGBTs.
 19. The power converter circuit of claim 1,further comprising an AC filtering circuit coupled to the load.
 20. Thepower converter circuit of claim 19, wherein the AC filtering circuitcomprises a capacitor bank having a plurality of capacitors formed in astar configuration.
 21. The power converter circuit of claim 20, whereinthe star configuration comprises a common node forming a neutral point.22. The power converter circuit of claim 19, further comprising a fifthregulator coupled to the AC filtering circuit.
 23. The power convertercircuit of claim 1, further comprising a transformer coupled between theinverter and the load for isolation.
 24. The power converter circuit ofclaim 1, further comprising a DC filtering circuit coupled between therectifier and the inverter.
 25. The power converter circuit of claim 24,wherein the DC filtering circuit comprises an LCL circuit or an LCcircuit.
 26. The power converter circuit of claim 1, wherein the firstAC voltage source is a three-phase AC voltage source having threephases, and wherein the resonant circuit includes three individualresonant circuits corresponding to the three phases respectively. 27.The power converter circuit of claim 26, wherein the rectifier includesthree individual rectifiers corresponding to the three individualresonant circuits respectively.
 28. The power converter circuit of claim27, wherein each of the three individual rectifiers comprises a diodebridge, a thyristor bridge, or a pulse-width modulation (PWM) circuit.29. The power converter circuit of claim 27, wherein the inverterincludes three individual inverters corresponding to the threeindividual rectifiers.
 30. The power converter circuit of claim 29,wherein each of the three individual inverters includes a thyristorbridge having four thyristors or a three-phase IGBT bridge having fourIGBTs.